The present invention, in some embodiments thereof, relates to a system, method and apparatus for measuring vascular parameters and, more particularly, but not exclusively, to a system, method and apparatus for monitoring changes in the equivalent inner diameter of small branching arteries and arterioles.
Many medical conditions are characterized by changes or abnormalities and size and shape of arterioles. Vasoconstriction and vasodilation are reversible changes in the diameters of arterioles. Vasoconstriction and vasodilation also play a role in regulating blood pressure, and in diseases characterized by abnormal regulation of blood pressure (hypertension and hypotension), general and peripheral blood flow impedance of subject, systemic vascular resistance (SVR). Other diseases are characterized by chronicle changes in the diameters and cross sections of arterioles, including diabetes and atherosclerosis.
Generally, arterioles are too small to image, using such imaging methods as ultrasound, MRI, and x-rays, including CT scans.
Other techniques for examining the circulatory system are known, for example. sphygmomanometry provides data on systolic and diastolic blood pressure, and pulse oximetry provides data on blood oxygen levels. Arterial line and central venous line sensors provide data on blood pressure and blood flow rate inside large blood vessels.
Josep Sola, Stefano F. Rimoldi, and Yves Allemann, “Ambulatory Monitoring of the Cardiovascular System: the role of Pulse Wave Velocity,” in New Developments in Biomedical Engineering, Chapter 21, p. 391-422, provides a review of techniques for measuring pulse wave velocity, primarily in large arteries over large distances, for example from the heart to the extremities.
WO2007/097702 discusses a method for the generation, detection and evaluation of a photoplethysmographic (PPG) signal to monitor blood characteristics, in which the light source(s) are spaced at particular distances from photodetector(s). U.S. Pat. Nos. 6,123,719, 5,891,022, US2009/0306487 and EP1297784 discuss photoplethysmographic measurement systems that have at least two light emitters, each emitting light at different wavelengths and a photodiode for detecting the intensity of light reflected from a patient's tissue such as blood, finger, etc.
US2010/0331708 describes methods for monitoring cardiovascular conditions, i.e., hyperblood flow related circulation, vasodilation, vasoconstriction, or central-to-peripheral arterial pressure decoupling conditions. These methods involve measuring a central signal proportional to or a function of the subject's heart activity and a peripheral signal proportional to or a function of a signal related to central signal. Then calculating a time or phase differences between features in the central and peripheral signals representing the same heart event. The cardiovascular condition is indicated if the time or phase difference is greater or lower than a threshold value over a specific period of time, or if there is a significant statistical change in the times over the specific time period. These methods can alert a user that a subject is experiencing some cardiovascular conditions, which can enable a clinician to appropriately provide treatment to the subject.
Said application provides methods, mostly suggesting estimation of common vasoconstriction or vasodilation level, by measurement between physiologically “central”, heart-related point and one of physiologically “peripheral” points of measured subject, actually providing estimations of blood flow impedance change along all branches of blood vessel tree, included between said two points. Additional background art includes Reuven Gladshtein, “Indications of cross-section of small branched blood vessels” WO 2012110955 A1, Minnan Xu, “Local Measurement of the Pulse Wave Velocity Using Doppler Ultrasound,” M. S. thesis, Dept. of Electrical Engineering and Computer Science, M.I.T., May 24, 2002; A. C. Fowler and M. J. McGuinness, “A Delay Recruitment Model of the Cardiovascular Control System,” submitted to Journal of Mathematical Biology, June 2004, revised December 2004; John Allen, “Photoplethysmography and its application in clinical physiological measurement,” Physiol. Meas. 28 (2007), R1-R39; H. S. Lim and G. Y. H. Lip, “Arterial stiffness in diabetes and hypertension,” Journal of Human Hypertension (2004) 18, 467-468; and Emilie Franceschini, Bruno Lombard, and Joël Piraux, “Ultrasound characterization of red blood cells distribution: a wave scattering simulation study,” Journal of Physics: Conference Series 269 (2011) 012014.